Communication from “fixed” Earth stations such as ground stations or ship-borne stations to rocket-propelled objects such as missiles is now common. The “Earth” station may even be an aircraft in this context. The communication is often in a military scenario in which the missile is defensive, and is commanded and or guided toward its target by signals transmitted from the Earth station. In one common scenario, a ship or ground station includes a radar or other sensing system which tracks both the target and the missile, and updates the information available to the missile guidance to improve the likelihood of hitting the target.
The quality of the communication link between the Earth station and the missile in a “link budget” which is determined by analysis that captures all of the hardware and environmental gains and losses of the communication path. Among the elements which can be incorporated into the link budget analysis during the boost phase or portion of the missile flight is rocket plasma plume radio-frequency (RF) attenuation, thought to be attributable to free electrons arising from alkali metal impurities found in the rocket propellant.
FIG. 1 illustrates a scenario 10 with a communication link, illustrated as a dash line 12, from a ground station 14 including an antenna 15. The communication link 12 passes through the plasma plume 16 of a missile 18 to get to the missile antenna 20. The plume is illustrated generally as 16, with various lines 16a, 16b, 16c, and 16d representing contours of different temperature ranges. The aspect angle Θ is illustrated as the angle between the RF path of the communication link 12 and the longitudinal axis 22 of the missile.
The radio-frequency (RF) link budget loss attributable to the plume is calculated by a complex and expensive modeling procedure. The calculation yields plume attenuation for an engine under various flight conditions, such as altitude, Mach number, and aspect angle.
FIG. 2 is a simplified representation of a prior-art control system scenario 210. In FIG. 2, the ground station 210 communicates with the antenna 20 of missile 18 by way of a bidirectional wireless link. The link may carry missile targeting information, including missile attitude commands and other telemetry. The current link quality or link budget is compared with a predetermined or precalculated plume attenuation in a microprocessor (μP) block 212. The communication link, 12 is monitored during flight of missile 18 to determine the link attenuation. The link attenuation is compared in block 212 with the predetermined plume attenuation information to determine if a significant part of the link attenuation is attributable to the plume. If the plume attenuation is determined to be significant, a missile track control block 214 adjusts the track of the missile, so that the communication link 12 does not pass through the plume 16, or passes through a lower-attenuation portion of the plume. This in turn eliminates or reduces the RF attenuation attributable to the passage of the RF through the plume. Thus, if the aspect angle at which the missile is “viewed” from the RF source is such that the RF signal passes through a portion of the plume that, in conjunction with other factors, occasions excessive attenuation or signal loss, the modification of the track is selected to move the missile track or path relative to the RF signal path so as to reduce the plume attenuation and thereby improve the link budget to provide reliable communications.
Improved methods are desired for determining link attenuation budgets in complex situations.